Add highlight in release note: .Net Core & Standard.

Author: fredericDelaporte

releasenotes.txt

@@ -3,6 +3,10 @@
 
 Release notes - NHibernate - Version 5.1.0
 
+** Highlights
+    * NHibernate has gained two new target frameworks: .Net Core 2.0 and .Net Standard 2.0. NHibernate NuGet package
+      provides them, along with the .Net framework 4.6.1 build.
+
     ##### Possible Breaking Changes #####
         * Since Ingres9Dialect is now supporting sequences, the enhanced-sequence identifier generator will default to
           using a sequence instead of a table. Revert to previous behavior by using its force_table_use parameter.